But that's just it, they don't. They convey the mood and atmosphere of a game table. I can't begin to imagine what the mood and atmosphere of the dungeon is by looking at a bunch of brightly colored plastic playing peices.
Oh, come now! You sure that's not the jaded gamer talking, and not the guy who set up the really cool street battle that was probably talked about by the players for a long time afterward?
For my experience, props for props' sake might not be that great, but the value of a good prop on the table always brings out a lot more fun than even a good description. "Show, don't tell" and "pictures are worth a thousand words" are aphorisms that have proven true time and again. Of the times I have gone to the trouble to set up good props (not as I say "props for props' sake, but something that actually added a tactile and appropriate element) it's always proven worth it. I don't do it all the time, because if you did it might get stale, but time to time it always brings out a little something that just descriptions don't do.
I once set up a home-made Coruscant street scene in a star wars game, during the Yuuzhan Vong invasion, consisting of some bridges, archways and stairs. The simple three-dimensional element, with the threat of PCs and Vong falling a thousand feet to their deaths, added a whole different element to the session, and led to a fall from grace for a Jedi in our group, as he was enjoying grabbing Vong with move object and tossing them off bridges FAR too much. (This was in the days before it was clarified what the Vong were and weren't immune to in the Force.)
A co-GM in his Star Wars game gave us three prop objects near the start of the campaign: a crystalline datapad, a broken sith holocron, and a the broken lightsaber of Mace Windu. Each came with a couple of surprises that were discovered by interacting with the props themselves in certain ways; it added a lot of fun to the game session, as each session we spent our spare time trying to figure out the significance of these props, and what they did.
There's a world of value to be gained from a good prop or puzzle, as long as it doesn't become a show-stopper or a sidetrack that takes you away from teh focus on your game itself. Gabe himself apparently made sure of this buy checking to be sure his light puzzle was solvable beforehand, and not too difficult. I give him kudoes for bringing the kind of enthusiasm to the table that a new player often does.